The present invention generally relates to a blade assembly for steam turbines, and more particularly to a blade assembly including a plurality of circumferentially aligned and spaced apart hollow blade members which have each a base and a head piece and are so arranged in the housing of the turbine as to form an inner ring and an outer ring.
Such blade assemblies are generally fabricated in two ways. In accordance with a first type of fabrication, the blade assemblies are made from malleable cast iron, and in a second mode of fabrication, the actual blade members are made from sheet metal and welded within outer and inner half-round rings. Blade assemblies made from malleable cast iron are characterized by a substantial weight and thus cost-intensive material use. Moreover, in order to keep predetermined tolerances, complex finishing works and required especially when using such high quality cast types as e.g. malleable cast iron. Still, despite thorough and careful grinding operation, the texture and surface of such components is inferior in comparison to ground surfaces of sheet metal components. As a consequence, also the corrosion-resistance is adversely affected and thus the longevity of the product. Compared to sheet metal constructions, cast components have another relevant drawback which resides in their inability to form a flow separation edge of sufficiently acute angle, so that their effectiveness considerably deteriorates.
Blade assemblies made by way of the second type of fabrication have also several drawbacks. The sequential welding of the blade members into the half-round rings results in significant tensions or distortions and dimensional variations that can only eliminated through complex operations because the outer and inner half-round rings absorb different amounts of energy, compounding imbalances as a result of the afore-stated problems. Apart therefrom, radial variations in dimensions by itself pose a problem.